Method of producing layered aluminum fine particles and use thereof

ABSTRACT

The present invention provides a method for producing layered aluminum fine particles, and applications to single electron tunneling quantum devices, and the present invention further relates to a method for producing spherical metallic aluminum fine particles (layered aluminum fine particles), characterized in that metallic aluminum is supplied into a mixed gas of helium and 1×10 −7  to 3×10 −7  torr water vapor by sputtering induced by argon gas discharge to generate aggregates, after which this product is released into a vacuum to generate single crystals in which the surface layer is covered with alumina.

This application is a Division of application Ser. No. 09/022,006 Filedon Feb. 11, 1998, now U.S. Pat. No. 6,146,505.

DISCLOSURE OF THE INVENTION

This invention relates to a convenient and efficient method forproducing spherical layered aluminum fine particles, the surface layerof which is covered with several nanometers of alumina, the interior ofwhich is metallic aluminum fine crystals, and which are useful for anelement of a quantum device that utilizes a single electron tunnelingphenomenon. More particularly, the present invention relates to a methodfor producing layered aluminum fine particles using a fine particlegenerating device that features magnetron sputtering; layered aluminumfine particles obtained by this method; and the application of theselayered aluminum fine particles to a single electron tunneling deviceelement.

BACKGROUND OF THE INVENTION

The production of elements on the sub-micron order is considered thelimit with today's semiconductor technology for increasing density andintegration, and some technique completely different from conventionalfine processing technology (such as lithography) is needed in order tocreate even finer elements. It is believed that if the elements of adevice could be smaller than sub-micron, then it would be possible tofabricate a quantum device (single electron tunneling device) that wouldcontrol individual electrons, rather than controlling the flow of groupsof electrons as in semiconductors produced up to now, and this hasbecome an area of fervent research of late (publication 1).

The layered aluminum fine particles obtained with the present inventionare single crystals of metallic aluminum whose surface is covered withan oxide insulator (alumina), and when these are placed on a substrate,it should be possible to create an electronic device based on singleelectron tunneling, which utilizes the passage of electrons one by onethrough this insulation layer (publication 2: an example is given ofsingle electron tunneling in metallic ultrafine particles placed on asubstrate). Therefore, these layered aluminum fine particles arebelieved to contribute greatly to obtaining an ultrafine semiconductorintended for a single electron tunneling quantum device. Mostconventional methods for manufacturing fine particles containingaluminum involved evaporation in a gas method (publication 3: thegeneration of ultrafine particles by evaporation in a dilute gas methodis discussed), but the fine particles obtained with these methods wereall fine particles of alumina, and there are no cases of the productionof layered aluminium fine particles that can be used for elements in aquantum device that utilizes single electron tunneling, such as areobtained with the present invention.

In view of this, in an effort to solve the above problem, the inventorsused a fine particle generating device featuring magnetron sputtering togenerate layered aluminum fine particles, and analyzed the particlesthus generated using a transmission electron microscope (TEM) andelectron beam diffraction. They also examined the dependence of particlesize on the shape of the fine particle generating source, thetemperature, and so on.

As a result, it was learned that when the distance from the aluminumsputtering target to the aperture attached to the distal end of thegenerating source is set at about 100 mm, and the aperture vicinity iscooled with liquid nitrogen, layered aluminum fine particles with adiameter of about 5 to 500 nm are generated through the aperture.

Furthermore, the inventors arrived at the present invention upondiscovering that when the vicinity of the outlet from the fine particlegenerating source is not cooled with liquid nitrogen, layered aluminumfine particles the same as those mentioned above can be generated byshortening the distance between the aluminum sputtering target and theaperture to about 30 to 50 mm.

SUMMARY OF THE INVENTION

The present invention provides a method for producing layered aluminumfine particles, and applications to single electron tunneling quantumdevices. The present invention further relates to a method for producingspherical metallic aluminum fine particles (layered aluminum fineparticles), characterized in that metallic aluminum is supplied into amixed gas of helium and 1×10⁻⁷ to 3×10⁻⁷ torr water vapor by sputteringinduced by argon gas discharge to generate aggregates, after which thisproduct is released into a vacuum to generate single crystals in whichthe surface layer is covered with alumina; a method for producinglayered aluminum fine particles with a particle diameter of 5 to 500 nm,characterized in that the above-mentioned layered aluminum fineparticles are placed in a flow of helium gas, released into a vacuum viaan aperture, and deposited directly onto a substrate; a method forproducing the above-mentioned layered aluminum fine microparticles,wherein the vicinity of the outlet from the particle generating sourceis cooled with liquid nitrogen, or the distance between the aluminumtarget and the aperture is set at 30 to 50 mm; layered aluminum fineparticles obtained by the above methods; and a single electron tunnelingquantum device element that makes use of layered aluminum fine particlesobtained by the above methods.

DETAILED DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a convenient andefficient method for producing metallic aluminum fine particles, withwhich single electron tunneling control is possible, and whose surfacelayer is an extremely thin insulation layer.

In order to solve the above problem, the present invention is such thatmetallic aluminum is supplied into a mixed gas of helium and 1×10⁻⁷ to3×10⁻⁷ torr water vapor by sputtering induced by argon gas discharge togenerate aggregates by impact with the helium yielding aluminum fineparticles (single crystals). The surface of the fine particles isoxidized into a surface layer of alumina through the action of the traceamount of water vapor contained in the helium gas here, and this is howlayered aluminum fine particles are produced. Also, the presentinvention is a method for producing layered aluminum fine particles witha particle diameter of 5 to 500 nm, characterized in that theabove-mentioned particles are placed in a flow of helium gas, releasedinto a vacuum via an aperture, and deposited directly onto a substrate.A preferred embodiment of this is a method for producing layeredaluminum fine particles in which the vicinity of the outlet from theparticle generating source is cooled with liquid nitrogen, or thedistance between the aluminum target and the aperture is set at 30 to 50mm. Other embodiments of the present invention are the layered aluminumfine particles obtained by the above method, and the application thereofto a single electron tunneling quantum device element.

The present invention will now be described in further detail.

In the present invention, metallic aluminum is supplied into a mixed gasof helium and 1×10⁻⁷ to 3×10⁻⁷ torr water vapor by sputtering induced byargon gas discharge to generate aggregates by impact with the heliumyielding aluminum fine particles. After this, the surface layer of themicroparticles is converted into alumina through the action of the traceamount of water contained in the helium gas, as a result of whichspherical fine particles are generated that are covered with severalnanometers of alumina surface layer, and that have metallic aluminumsingle crystals in the interior. A method for producing fine particlethat makes use of a sputter gun is an effective method for producing anactual thin film sample because it allows a large quantity of fineparticles to be generated continuously (magnetron sputtering gasaggregation method). This has been proven in Germany by Helmut Haberland(H. Haberland, M. Moseler, Y. Qiang, O. Rattunde, Y. Thurner, and Th.Reiners: Proceedings of the Conference on Beam Proceedings of AdvancedMaterials, Cleveland, Ohio, USA, pp. 1-7 (1995)).

FIG. 1 is a concept diagram of the fine particle source. As shown in thefigure, the magnetron gun is positioned inside a stainless steel tube.An aperture (1 to 5 mm in diameter) is attached to the distal end ofthis tube, and the generated fine particles are taken out through thisaperture.

The distal end portion of the particle generating source can be cooledwith liquid nitrogen in order to lower the temperature of the heliumgas.

Aluminum atoms are sputtered from the metallic aluminum target (a disk50 mm in diameter and 3 mm thick) by direct current discharge of argongas (4×10⁻³ torr). Helium gas (10 torr) fills the area around thesputter gun, and aggregation occurs in this area as a result of impactwith the helium gas, thus generating aluminum fine particles. Thesurface of these fine particles is oxidized into alumina by the traceamount (about 1×10⁻⁷ to 3×10⁻⁷ torr) of water vapor present in thehelium gas, resulting in the generation of spherical layered aluminumfine particles in which the surface layer is covered by alumina with athickness of 1 to 5 nm, and the interior is metallic aluminum singlecrystals. The helium gas also serves to push the fine particles out ofthe particle generating source and into the vacuum tank. The stainlesssteel tube is position in the middle of the chamber, and the distancefrom the aluminum target to the outlet from this tube is 30 to 150 mm.The chamber is evacuated with an oil diffusion pump (5000 L/sec).

In the present invention, the above apparatus and conditions are givenas favorable examples, and sputtering conditions include an argon gaspressure of 4×10⁻³ to 10 torr, a direct current discharge of 100 to 600V and 0.1 to 0.5 A, an aluminum target diameter of 30 to 50 mm, a heliumgas pressure of 0 to 10 torr, and a water vapor pressure of 1×10⁻⁷ to3×10⁻⁷ torr. Almost no oxide film will be formed on the particle surfacelayer if the water vapor pressure is below the range given above, and itis undesirable for the above range to be exceeded because the metallicaluminum in the particle interior will all be oxidized. A favorableexample of an apparatus is one in which the diameter of theabove-mentioned aperture is 1 to 5 mm and the metal tube is made ofstainless steel, copper, or the like.

Favorable examples of the substrate on which the layered aluminum fineparticles are deposited include copper, aluminum, another such metalsand silicon and other semiconductors, while favorable configurationsthereof include substrates of single crystals and polycrystals.

The distance from the fine particle generating source to the substrateis about 10 mm. The vicinity of the outlet from the fine particlegenerating source can be cooled with liquid nitrogen to between −60° C.and −180° C.

The energy required when the generated fine particles are placed in aflow of helium gas, released into a vacuum through the aperture, anddeposited directly on the substrate is 0.05 eV or less per atomconstituting the fine particles. Accordingly, the fine particles can bedeposited on the substrate without their layered structure of sphericalstructure being altered.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a fine particle generating device in which magnetronsputtering is used;

FIG. 2 is a TEM image of the layered aluminum fine particles that aregenerated;

FIG. 3 is a TEM image of the surface layer portion of the layeredaluminum fine particles;

FIG. 4 is the electron diffraction pattern (particle structure) of theinterior of the layered aluminum fine particles; and

FIG. 5 is the electron diffraction pattern (particle structure) of thesurface layer of the layered aluminum fine particles.

EXAMPLES

The present invention will now be described on the basis of examples,but the present invention is not limited by these examples.

Example

(2) Generation of Layered Aluminum Fine Particles

Aluminum atoms were sputtered from an aluminum target (50 mm diameter)by the direct current discharge (220 V, 0.4 A) of argon gas (99.99%purity, 4×10⁻³ torr) introduced by a mass flow meter, these atoms weresupplied to a mixed gas of helium gas (99.99% purity, 10 torr) and watervapor (1×10⁻⁷ torr) introduced by a mass flow meter, and the aluminumatoms were made to aggregate in this atmosphere to generate metallicaluminum fine particles, after which the particles were released into avacuum through an aperture (2 mm diameter). In this case, the vicinityof the outlet from the particle generating source was cooled with liquidnitrogen, and the distance between the aperture and the aluminum targetwas 100 mm. Cooling of the vicinity of the outlet from the particlegenerating source with liquid nitrogen to between −60° C. and −180° C.allowed the amount of water vapor inside the generating source to be setat 1×10⁻⁷ to 3×10⁻⁷ torr, which is optimal for the generation of layeredaluminum fine particles.

Furthermore, it was found that when the vicinity of the outlet from thefine particle generating source is not cooled with liquid nitrogen,layered aluminum fine particles the same as those mentioned above canstill be generated by shortening the distance between the aluminumtarget and the aperture to about 30 to 50 mm. This makes it possible toprevent the generated aluminum metal fine particles from reacting withexcess water vapor and becoming alumina.

In order to observe the fine particles by TEM, a copper mesh equippedwith a carbon thin film was exposed to a fine particle beam for 30 to 60minutes. The distance between the fine particle source outlet and themesh was 10 mm. This sample was used to analyze the fine particlesdeposited on the mesh by TEM (100 kV and 200 kV (Hitachi HF-2000)).

(2) Analysis of Layered Aluminum Fine Particles by TEM

FIG. 2 is a TEM image of the layered aluminum fine particles that weregenerated. These particles were generated at normal temperature and withthe distance between the aluminum target and the aperture set at 30 mm.As is clear from the figure, the diameter of the particles was found tobe approximately 120 nm, and the shape thereof to be spherical. It canalso be seen that the surface layer thereof consists of a layer severalnanometers in thickness that is different from the center portion. FIG.3 is a TEM image in which the vicinity of the surface layer of thegenerated fine particles has been enlarged. It can be seen that thesurface layer and the center portion are distinctly separate.

Next, FIG. 4 is the electron diffraction pattern from the center portionof the fine particles. It was found that the diffraction spots appearingin the figure can be explained as diffraction spots from the (111) planeof aluminum fcc single crystals (publication 4). It was concluded fromthese results that the center portion of the generated fine particleswas single crystals of metallic aluminum.

FIG. 5 is the electron diffraction pattern of the surface layer of thegenerated fine particles. Because the thickness of the surface layer isonly a few nanometers, it is difficult to measure the diffractionpattern for just this portion, but it was found that the diffractionspots appearing in the figure can be explained as diffraction spots fromthe fcc (111) plane of aluminum single crystals, and diffraction spotsfrom γ-alumina (publication 4). It was concluded from these results thatthe surface layer of the generated fine particles was γ-alumina.

It can be seen from the above results that the generated aluminum fineparticles were spherical aluminum single crystal fine particles (layeredaluminum fine particles) in which the surface layer was covered withγ-alumina in a thickness of a few nanometers (1 to 5 nm).

Because the layered aluminum fine particles generated with the presentinvention have an interior composed of conductive metallic aluminum anda surface layer composed of insulating alumina, a microscopic capacitorcan be constructed by bringing a metal into contact with the surface ofthe fine particles. With a nanometer-scale capacitor such as this, thepassage of a single electron through the insulation layer (singleelectron tunneling) can be observed, which means that it is possible tomake a quantum device such as a transistor that utilizes this effect(publications 1 and 2). Electronic devices such as this will likely beof great import in the future, and as can be seen from the above, thelayered aluminum fine particles generated with the present inventionshould be usable for elements in devices such as this.

For layered aluminum fine particles to be utilized as quantum deviceelements, it is important that the layered structure thereof (astructure in which the metallic aluminum is covered with alumina) bepreserved after deposition onto the substrate. As is clear from theabove-mentioned TEM images, with the method of the present invention thefine particles are deposited on the substrate with their spherical shapestill in tact. This is because the generated fine particles are placedin a flow of helium gas, released into a vacuum, and deposited on thesubstrate at a low energy level of about 0.03 eV (no more than 0.05 eV)per constituent atom of the fine particles.

To summarize the above, it was found that layered aluminum fineparticles are generated by a magnetron gas aggregation method. On thebasis of TEM images and electron beam diffraction analysis, it was foundthat the generated fine particles are spherical in shape, with adiameter of about 5 to 500 nm, and have a two-layer structure in whichthe surface layer is composed of γ-alumina and the interior of singlecrystals of metallic aluminum.

It was also found that layered aluminum fine particles can be generatedby either of two types of method, namely, setting the distance betweenthe aluminum target and the particle generating source outlet to about100 mm and cooling the vicinity of the outlet with liquid nitrogen, ornot cooling with liquid nitrogen, and shortening the distance betweenthe target and the outlet to about 30 to 50 mm.

It was proven that the use of helium gas in the conveyance of the fineparticles and their deposition onto the substrate allows the fineparticles to be deposited onto the substrate with their spherical shapein tact at a low energy level of no more than 0.05 eV per constituentatom of the fine particles.

As detailed above, metallic aluminum is supplied into a mixed gas ofhelium and 1×10⁻⁷ to 3×10⁻⁷ torr water vapor by sputtering induced byargon gas discharge to generate aggregates by impact with the heliumyielding aluminum fine particles (single crystals). The surface of thefine particles is oxidized into a surface layer of alumina through theaction of the trace amount of water vapor contained in the helium gashere, and this is how layered aluminum fine particles are produced.Also, the present invention pertains to a method for groducing layeredaluminum fine particles with a particle diameter of 5 to 500 nm,characterized in that the above-mentioned particles are placed in a flowof helium gas, released into a vacuum via an aperture, and depositeddirectly onto a substrate; a method for producing the abovementionedlayered aluminum fine particles, in which the vicinity of the outletfrom the particle generating source is cooled with liquid nitrogen, orthe distance between the aluminum target and the aperture is set at 30to 50 mm; or the like. The following merits are realized with thepresent invention.

(1) Layered aluminum fine particles with a diameter of 5 to 500 nm,which are useful for a quantum device element that utilizes singleelectron tunneling, can be generated efficiently.

(2) Low-energy layered aluminum fine particles can be deposited onto asubstrate with their layered structure and their spherical shape stillin tact. As a result, these fine particles can be utilized efficientlyas quantum device elements.

Reference Publications

1. S. Ueda: Oyo Butsuri, 62, 889 (1993).

2. R. Wilkins, E. Ben-Jacob, and R.C. Jaklevic: Phys. Rev. Lett., 63,801 (1989).

3. Solid Physics and Metal Physics Seminars Special Edition, “UltrafineParticles,” p. 68, “5. Generation of Ultrafine Particles,” AgneTechnology Center (1984)

.4. S. Wilson and J. McConnel, Solid State Chem., 34, 315 (1980).

What is claimed is:
 1. Spherical metallic fine particles with diametersof 5 to 500 nm of single crystal aluminum having a surface layer ofalumina obtained by a method which comprises: (a) generating aluminumfrom an aluminum target by argon gas discharge-induced sputtering; (b)introducing said aluminum into a mixed gas of water vapor at a pressureof 1×10⁻⁷ to 3×10⁻⁷ torr, and helium to generate aluminum particlesaggregated; and (c) introducing the particles into a vacuum to generatethe single crystal aluminum having a surface layer of alumina. 2.Spherical metallic fine particles with diameters of 5 to 500 nm ofsingle crystal aluminum having a surface layer of alumina deposited ontoa substrate, obtained by a method which comprises introducing theparticles obtained by the method of claim 1 into a vacuum via anaperture by a flow of helium gas to deposit the particles directly ontothe substrate.
 3. The spherical metallic fine particles of singlecrystal aluminum having a surface layer of alumina and having a particlediameter of 5 to 500 nm deposited onto the substrate of claim 2, whereineither a vicinity of the aperture is cooled with liquid nitrogen, or adistance between the aluminum target and the aperture is set at adistance from 30 to 50 nm.
 4. A single electron tunneling quantum deviceelement containing the spherical metallic fine particles of singlecrystal aluminum having a surface layer of alumina and having a particlediameter of 5 to 500 nm obtained by the method of claim 1 and a metalbeing in contact with the surface of the fine particles.
 5. A singleelectron tunneling quantum device element containing the sphericalmetallic fine particles of single crystal aluminum having a surfacelayer of alumina and having a particle diameter of 5 to 500 nm depositedonto a substrate obtained by the method of claim 2 and a metal being incontact with the surface of the fine particles.
 6. A single electrontunneling quantum device element containing the spherical metallic fineparticles of single crystal aluminum having a surface layer of aluminaand having a particle diameter of 5 to 500 nm deposited onto a substrateobtained by the method of claim 3 and a metal being in contact with thesurface of the fine particles.